Before, it was not possible at all to copy the text from the agent
panel, only its markdown.
The PR breaks the existing ctrl/cmd-c binding there by using the same
approach markdown preview does: copy as text by default, allow to copy
as markdown via the context menu.

Before:


https://github.com/user-attachments/assets/2014f363-5167-4700-be86-9c0774d7e4fd

After:


https://github.com/user-attachments/assets/1dad253c-7a3b-4ca7-99c8-e8df8b3c3fd6


Release Notes:

- Changed agent panel to copy text with cmd/ctrl-c, as markdown preview
does. `markdown::CopyAsMarkdown` action is available still and can be
bound if needed.